Study of leakage current mechanisms in thin film dielectrics on 4H-SiC

June 17, 2015 @ 3:00 pm - 4:00 pm

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SiC have excellent physical and electronic properties such as high breakdown electric field, high band gap and high thermal conductivity. Along with this SiC is the only compound semiconductor to have a native oxide (SiO2) on it. These properties have made SiC a material of choice for Metal-Oxide-Semiconductor (MOS) devices operating in high temperature and high power regions.But there are serious challenges for SiC based MOS devices to realize its full potential. One of them is the effective barrier height (ΦB) between the SiC Fermi level and SiO2 conduction band edge which is low, compared to ΦB of silicon (Si) and SiO2 leading to large amount of tunneling current through its oxide layer. Moreover, ΦB shows a decreasing trend with increase in temperature questioning the reliability of SiC based devices at higher operating temperature. High interface state density (Dit) at SiC/SiO2interface also lowers the ΦB leading to high tunneling current.

Though, SiO2 is the most popular and widely studied gate dielectric on SiC, it faces challenges during high gate electric field applications. The dielectric constant of SiO2 (3.9) being low compared to 4H-SiC (10) results in an electric field in SiO2 which is 2.5 times higher than in SiC resulting in its premature breakdown. So, the high breakdown electric field of SiC (3 MV/cm) which is an asset in high power application is underutilized. This limitation of SiO2 has prompted researchers to investigate the use of high-k dielectrics on SiC.

Our work is divided into two parts. In first part, we have studied the presence of direct tunneling (DT) current in ultra thin oxide (SiO2) on 4H-SiC and its dependence on temperature and SiC/SiO2 interface quality. At higher temperatures Schottky Emission (SE) current also influences the gate leakage current in ultra thin oxide on 4H-SiC. In this work 4H-SiC is thermally oxidized in high water vapor pressure because increasing oxidation pressure reduces thermal budget of SiC oxidation which is very high.In the second part of our work, we investigated the use of reactive-ion-sputtered (RIS)-Al2O3 as high-k dielectrics on 4H-SiC. We studied the effect of O2 annealing on the improvement of gate leakage current through the RIS-Al2O3 and the subsequent improvement in the C-V hysteresis by forming gas (95% N2 and 5% H2) annealing.